Google counts 5,000 a day per domain, not per mailbox
This is the number most senders miss, because every tool they use counts something else.
The arithmetic that catches people out
Twenty mailboxes on one domain, each sending twenty a day, is four hundred messages. Every per-mailbox limit says that is fine, because twenty is a small number. Scale the same setup to two hundred and fifty mailboxes — a normal agency rollout — and the domain is sending five thousand a day while each individual mailbox still looks healthy.
Google aggregates across the primary domain. A subdomain counts too:
mail.example.com and example.com are one sender.
What changes when you cross it
Crossing is not a punishment and not itself a failure. It changes which rules are enforced rather than advised:
SPF, DKIM and DMARC alignment. Not just present — aligned. A DMARC record
of p=none satisfies the requirement; failing alignment does not.
One-click unsubscribe. The List-Unsubscribe-Post header,
RFC 8058, honoured within two days.
A spam complaint rate under 0.3%, and ideally under 0.1%. This is the one that ends senders. It is measured by recipients pressing a button, which no amount of authentication protects you from.
The penalty shifts from filtering to rejection. A filtered message is in a folder somebody might open; a rejected one never arrived and you learn nothing useful from the bounce.
Why the classification is worth planning for
Google keeps the bulk-sender classification once a domain reaches the threshold. That is not a reason to avoid volume — it is a reason to have authentication in order before you get there, because doing it afterwards means fixing DNS while mail is already being rejected. DNS propagation alone takes hours.
How to actually count it
Sum every message sent in the last twenty-four hours, grouped by the domain of the sending address, rolling subdomains up into their parent. Count only what left — queued and failed messages are not volume anyone saw. Quotarider reports this on the mailbox page and warns at 80%, while there is still time to change something.
The 16 rules that put an email in spam →
Quotarider keeps the touch, the reply, the deal and the paid invoice in one database, behind a deliverability guardrail. Free plan, no card.